In the preparation of polymers of vinyl aromatic monomers, particularly styrene, with maleic anhydride or similar unsaturated anhydrides, by mass or solution polymerization, devolatilization of unreacted monomer, solvent and other volatile components is an absolutely vital step. As polymer technology has progressed, the acceptable level of residual volatile material, particularly as monomer, in a polymer has decreased. While low residual monomer is important for all commercial applications, it is of particular importance in food contact applications, such as containers, where taste and odor contributions to the food are found at relatively low levels. Lower volatile contents also result in improved thermal and mechanical properties. Various means have been employed in the past to reduce volatile content in commercial polymers. However it has not been possible to totally eliminate the volatile content of such polymers.
Multi-polymers, i.e., polymers involving more than one monomer, such as polymers of styrene and maleic anhydride are of particular interest for food contact applications. This interest comes from the increase in heat resistance over polystyrene that results from copolymerization with maleic anhydride. For each added percent of maleic anhydride the heat distortion temperature of the copolymer is increased by nearly 2° C. (Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 315-321).
Street in U.S. Pat. No. 3,536,787 noted that rubber modified, high impact polystyrene containing 3-5% unreacted styrene could be devolatilized to 2000 ppm of styrene monomer in an extruder devolatilizer system. Street then demonstrated that injection of 0.1% water as a stripping agent, prior to the extruder, lowered the styrene content to 1000 ppm. Generally polymerization to 95-97% conversion level disclosed by Street is not practical because of the long polymerization time required. Further, an extruder devolatilization system is not considered practical for large-scale commercial operations. High capital and operating cost as well as operational problems with vacuum line plugging are among major deterrents to their use.
Fujimoto, in U.S. Pat. No. 3,987,235, discloses a devolatilization by introducing methanol into molten styrene polymer and removing the volatiles under vacuum. However, this route requires a high pressure apparatus for dispersing the methanol into the polymer and an entire additional devolatilization system. This additional system includes a polymer heater, a vacuum chamber with expulsion pump, condenser and other auxiliary equipment.
Skilbeck has shown in U.S. Pat. No. 5,380,822 (which is hereby incorporated by reference to demonstrate the mechanical equipment and steps employed in devolatilization) that a previously devolatilized molten blend of polystyrene and rubber modified polystyrene containing less than 2 weight % residual monomer could be further reduced by dispersing water as a stripping agent into the molten stream and passing it through a vacuum devolatilization system.
Maleic anhydride, MA, can be used as a comonomer to increase heat resistance of polystyrene, but requires intensive mixing during copolymerization. Moore (Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 315-321) teaches conditions to produce homogeneous copolymers of styrene and 0 to 33% of MA. Moore later demonstrated (U.S. Pat. No. 3,919,354) that high impact copolymers of styrene and MA could be produced in a series of reactors. These copolymers all have Vicat softening points that are about 2° C. above that of homopolymer polystyrene for each added percent of MA.
High degrees of agitation are necessary in order to produce homogeneous copolymers of styrene and MA. If agitation is gradually reduced the first indication of a lack in homogeneity will be a slight haze. Further reduction will produce a cloudy copolymer and a still further reduction will result in an opaque copolymer. In a crystal grade copolymer clarity is of value for most applications. A haze will thus reduce the value of copolymer for most unpigmented applications. Cloudiness will reduce value further and there are relatively few commercial applications that will accept an opaque polymer. Rubber modified styrene/maleic anhydride copolymers are more tolerant of non-homogeneous polymer but will see a loss in impact strength when there is a high degree of non-homogeneity. It is desirable to produce a heat resistant styrene copolymer or terpolymer or rubber modified heat resistant polymer with reduced agitation requirements.